Approaches to Line Balancing COMSOAL & RPW

1. Approaches to Line BalancingCOMSOAL & RPW Active Learning Module 2 Dr. César O. Malavé Texas A&M University

2. Background Material • Modeling and Analysis of Manufacturing Systemsby Ronald G. Askin , Charles R. Standridge, John Wiley & Sons, 1993, Chapter 2. • Manufacturing Systems Engineering by Stanley B. Gershwin, Prentice – Hall,1994, Chapter 2. • Any good manufacturing systems textbook which has detailed explanation on reliable serial systems.

3. Lecture Objectives • At the end of this module, the students should be able to • Explain the approaches to line balancing • COMSOAL Random Sequence Generation • Ranked Positional Weight Heuristics • Solve and find the optimal solutions to line balancing problems using the above techniques

4. Introduction 5 Readiness Assessment Test (RAT) 5 COMSOAL Procedure 12 Spot Exercise 5 RPW Procedure 15 Team Exercise 5 Assignment 3 Total Time 50 Mins Time Management

5. Readiness Assessment Test (RAT) 1. In a layout, work stations are arranged according to the general function they perform without regard to any particular product. a) product, b) process, c) fixed position, d) storage 2. A product layout is more suited to situations where product demand is stable than when it is fluctuating. a) True, b) False 3. Fixed position layouts are used in projects where the product cannot be moved, and therefore equipment, workers, and materials are brought to it. a) True, b) False 4. In general, work-in-process inventory is large for a product layout and small for a process layout. a) True, b) False 5. Which of the following characteristics is associated with process layout? a) stable demand b) less skilled workers c) specialized machinery d) low volume e) product for general market

6. RAT – Solution • In a Process layout, work stations are arranged according to the general function they perform without regard to any particular product. • True. A product layout is more suited to situations where product demand is stable than when it is fluctuating. • True. Fixed position layouts are used in projects where the product cannot be moved, and therefore equipment, workers, and materials are brought to it. • False. In general, work-in-process inventory is large for a process layout and small for a product layout. • Low Volume is associated with process layout.

7. Approaches to Line Balancing Three Basic Approaches for finding a solution • COMSOAL – Basic random solution generation method • Ranked Positional Weight Heuristic – Good solutions found quickly • Implicit Enumeration Scheme Assumptions Required cycle time, sequencing restrictions and task times are known

8. COMSOAL Random Sequence Generation • A simple record-keeping approach that allows a large number of possible sequences to be examined quickly • Only tasks that satisfy all the constraints are considered at each step. • Sequence discarded as soon as it exceeds the upper bound. • Sequence saved if it is better than the previous upper bound and the bound is updated. • Efficiency depends on the data storage and processing structure

9. COMSOAL – Cont… • COMSOAL uses several list for speed computation. • NIP(i) Number of immediate predecessors for each task i. • WIP(i) Indicates for which other tasks iis an immediate predecessor. • TK Consists of N tasks. • During each sequence generation, • List of unassigned tasks (A) • Tasks from A with all immediate predecessors (B) • Tasks from B with task times not exceeding remaining cycle time in the workstation (F – Fit List) are updated.

10. COMSOAL Procedure • Set x = 0, UB = , C = Cycle Time, c = C. • Start the new sequence : Set x = x+1, A = TK, NIPW(i) = NIP(i). • Precedence Feasibility : For all, if NIPW(i) = 0, add i to B. • Time Feasibility : For all i B, if ti≤ c, add i to F. If F empty, Step 5; otherwise Step 6. • Open new station : IDLE = IDLE + c. c = C. If IDLE> UB go to Step 2; Otherwise Step 3.

11. COMSOAL Procedure – Cont… • Select Task : Set m = card {F}. Randomly generate RN  U(0,1). Let i* = [m.RN]th task from F. Remove i* from A, B, F. c = c – ti*. For all i  WIP(i*), NIPW(i) = NIPW(i) – 1. If A empty, go to Step 7; otherwise go to Step 3. • Schedule completion : IDLE = IDLE + c. If IDLE ≤ UB, UB = IDLEand store schedule. If x = X, stop; otherwise go to Step 2.

12. COMSOAL – Advantages • The technique is relatively easy to program. • Feasible solutions are found quickly. • Greater the computational effort expended, the better the expected solution . • Basic idea can be applied to many decision problems, the only requirement being that we can build solutions sequentially and a function evaluation can be performed to rank candidate solutions.

13. COMSOAL – Example

14. COMSOAL – Example Data Known : • Two 4 hour-shifts, 4 days a week will be used for assembly. • Each shift receives two 10 minute breaks. • Planned production rate of 1500 units/week. • No Zoning constraints exist.

15. Example Solution Model Car Precedence Structure 20 5 5 6 a b c j 21 15 10 d g h 8 15 46 16 e i k l 35 f

16. Example Solution – Cont… • To meet demand C = 70 Seconds. • Initially four potential tasks a, d, e, or f • Generate random number between 0 and 1. Say outcome in our case is 0.34 • R is in second quadrant so keep d as first task. • Continue the random generation. • Quick check of lower bound Thus Better Solutions may exist

17. Single COMSOAL Sequence Results

18. Solve the following line balancing problem using COMSOAL procedure. Assume demand is 100/day. Spot Exercise

19. Exercise Solution 1 b 2 3 1 3 a d e f 2 c

20. Exercise Solution – Cont…

21. Ranked Positional Weight Heuristic • A task is prioritized based on the cumulative assembly time associated with itself and its successors. • Tasks are assigned in this order to the lowest numbered feasible workstation. • Cumulative remaining assembly time constrains the number of workstations required. • Illustrates the greedy, single pass heuristics. • Procedure requires computation of positional weightPW(i) of each task.

22. RPW Procedure • Let S(i) Set of successors of tasks i. • Example, j  S(i) means j cannot begin until iis complete. • Compute PWi = ti + • Tasks ordered such that i < r implies i not  S(r). • Task r is then a member of S(i) only if there exists an immediate successor relationship from ito r. • Immediate successors IS(i) are known from the inverse of the IP(i) relationships.

23. RPW Procedure – Cont… • Task Ordering : For all tasks i = 1,…,N compute PW(i). Order (rank) tasks by nonincreasing PW(i) • Task Assignment: For ranked tasks i = ,…,N assign task i to first feasible workstation. Precedence Constraints : assignment to any workstation at least as large as that to which its predecessors are assigned Zoning & Time Restrictions : Checked on placement.

24. RPW Procedure - Example

25. Example Solution Model Car Precedence Structure 20 5 5 6 a b c j 21 15 10 d g h 8 15 46 16 e i k l 35 f

26. RPW Procedure - Solution Positional Weight calculated based on the precedence structure (previous slide). PWl = its task time = 16 PWk= tk + PWl = 46+16 = 62 PWj = tj + PWk = 5+62 = 67

27. Station Time Remaining Tasks 1 70, 50, 29, 23, 18, 3 a, d, b, c, g 2 70, 35, 25, 17, 2 f, h, e, i 3 70, 65, 19, 3 j, k, l RPW Solution Cont… • Assignment order is given by the rankings. • Task a assigned to station 1. • c - ta = 70 – 20 = 50 seconds left in Station 1. • Next Assign task d • 50 – 21 = 29 seconds left in Station 1.

28. Task Time Immediate Predecessors a 20 - b 18 - c 6 a d 10 a e 6 b f 7 c, d g 6 e, f h 14 g Team Exercise Assembly of a product has been divided into elemental tasks suitable for assignment to unskilled workers. Task times and constraints are given below. Solve by RPW Procedure

29. 6 c 7 f 20 10 10 6 14 a d g h 18 6 b e Exercise Solution

30. Assignment Write a flowchart for COMSOAL using the decision rule that feasible tasks are selected with probability proportional to their positional weight.